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The Greenland Cod (Gadus Morhua) at Iceland 1941-90 and Their Impact on Assessments

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The Greenland Cod (Gadus Morhua) at Iceland 1941-90 and Their Impact on Assessments NAFO Sci. Coun. Studies, 18: 81–85 The Greenland Cod (Gadus morhua) at Iceland 1941–90 and Their Impact on Assessments Sigfús A. Schopka Marine Research Institute, Skúlagata 4 121 Reykjavik, Iceland Abstract Studies have shown cod (Gadus morhua L.) in Icelandic waters form a unit stock with very little emigration, while cod from East and West Greenland migrate to Iceland and affect the stock abundance there. Such immigrations have through recent history resulted in the overestimating of the Iceland stock size while underestimating fishing mortality. Noting that for Icelandic cod the catch-at-age peaks at ages 4 or 5 years while immigrations from Greenland causes it to peak again at 7–9 years, Virtual Population Analysis was carried out on Icelandic stock analysis data from the period 1941–90 to determine years where fishing mortalities were higher for the older age groups. Immigrating year-classes through the period were thereby identified, particularly the outstanding 1945 year-class. On the other hand, it was observed that some year-classes emigrating from Greenland had in fact originated from spawning grounds at Iceland and drifted as egg/larvae to Greenland. This demonstrated the importance of information on larval drift. Key words: Cod, Gadus morhua, exploitation pattern, fishing mortality, Iceland–Greenland, migrational rate, stock assessment Introduction mortalities for those year-classes at Iceland which are subject to such an immigration. Tagging experiments at Iceland show that cod (Gadus morhua L.) in Icelandic waters are more or Materials and Methods less a unit stock (Jónsson, 1990). From more than 100 000 cod tagged since 1948, less than 30 recap- For this analysis, data on catch-in-numbers were tures have been recorded outside the Icelandic available dating back to 1941. Data for the most fishing grounds (J. Jónsson, pers. comm.). On the recent years, i.e. 1971 and later, were basically the other hand, it is well known that cod from Greenland same as in Anon. (1991) with some modifications waters, both West and East Greenland stocks, do for cod older than 14 years. For the period 1955–70 migrate in some years even on a large scale to basic data were the same as in Anon. (MS 1976) but Iceland (Hovgård et al., MS 1989; Harden Jones, data on the Icelandic fisheries were revised based 1968). These immigrations affect the stock abun- on detailed information on how catches were dis- dance at Iceland and hence the management of the tributed by gear and area. Also, mean weights-at- stock. It is therefore important, in order to under- age have been revised to obtain more reliable SOP- stand the fluctuations in the stock at Iceland, to be figures than those which can be calculated from the able to distinguish between the two cod stock com- data in Anon. (1976). The SOP-values between ponents, i.e. the Icelandic and the Greenlandic 1955 and 1970 give much higher landings than in components. reality. Therefore, mean lengths-at-age available for different fisheries were converted to mean In general there is no exact solution available to weights-at-age and then weighted by the catch of the problem at present. Theoretically tagging ex- each fishery to give the final figures of the total periments would solve this under the assumption fisheries. The revised mean weights were then used that all recaptures were available for analysis and for the stock biomass calculations in the above- that tagging mortality, fishing mortality and natural mentioned period. Estimated catch-in-numbers mortality for both components are known. As these for the period 1941–52 were entirely based on assumptions are never met, analysis of tagging Icelandic age-length data available at MRI but in experiments have only provided average estimates some cases never been published or worked up (Anon., MS 1981). Other parameters which are until recently (Schopka in preparation). Prior to different for these stocks, such as otolith structure 1953, only length composition data were available (Rätz, MS 1990), growth rate etc., have not yet for the German fisheries but age-composition data answered this question sufficiently. were also available for the period 1953–54. By using the Icelandic age-length keys, the German In this paper an attempt is made to analyze the length composition data were used to account for immigration by studying the changes in the fishing that fishery. 82 Sci. Council Studies, No. 18, 1993 Another important cod fishery at Iceland in those TABLE 1. Cod at Iceland: F-at-age from an ordinary years was that of the United Kingdom (UK). The VPA run for the 1960 to 1964 year-classes. only available information on that fishery prior to 1955 were catch and effort data by fishing grounds Year-class and by time of year. Icelandic age-composition Age 1960 1961 1962 1963 1964 data for the same time and grounds were then used to convert the UK landings into catch-in-numbers 3 0.12 0.08 0.10 0.08 0.09 data. 4 0.28 0.17 0.20 0.16 0.25 5 0.41 0.16 0.24 0.19 0.40 6 0.42 0.13 0.25 0.22 0.40 For the cod fisheries at Iceland catch-by-age 7 0.52 0.27 0.31 0.30 0.52 normally reaches a peak at age groups 4 or 5 years. 8 1.08 0.44 0.57 0.38 0.60 For a typical Greenland year-class entering Iceland 9 0.76 1.05 0.60 0.70 0.68 grounds, catch in numbers peaks to fish at age 7 or 10 0.69 1.11 1.03 1.11 1.04 8 years, or even 9 years in the case of a strong year- class such as the 1945 year-class (Fig.1). From the available catch-in-numbers data an ordinary Virtual TABLE 2. Cod at Iceland: F-at-age from a VPA run Population Analysis (VPA) was run. Input fishing where F-values have been in- creased mortality (F) for the oldest age group was chosen as to account for the im-migration of 1961– 63 year-classes. 0.5 but terminal Fs in 1990 were the same as in Anon. (1991). It turned out that, as expected for Year-class some year-classes, Fs for the younger age groups were extremely low. Based on the assumption that Age 1960 1961 1962 1963 1964 the exploitation pattern had not changed, it was 3 0.12 0.11 0.12 0.10 0.09 expected that Fs for these age groups should not be 4 0.28 0.25 0.24 0.23 0.25 lower than for the surrounding year-classes at same 5 0.41 0.27 0.31 0.30 0.40 age and the difference can be explained as an 6 0.42 0.24 0.35 0.40 0.40 effect of an immigration. To account for this, F on 7 0.52 0.60 0.50 0.50 0.52 these age groups can be increased. In the present 8 1.08 0.70 0.70 0.70 0.60 paper, F was increased to a level of a similar order 9 0.76 1.05 0.80 0.70 0.68 as that of the surrounding year-classes at the same 10 0.69 1.11 1.03 1.11 1.04 age, or to a level representing the likely fishing pattern. An example of the method for the 1960–64 year-classes is shown in Tables 1 and 2. It should data or effort data are available, F values can be be noted that the underlined F-values in Table 2 selected by tuning (Gunnar Stefánsson, Marine Re- were chosen as input values. If groundfish survey search Institute, Iceland, pers. comm.). In the case of Icelandic cod, such data were only available for the most recent years. It can of course be argued that the F-values chosen here are not the right ones, but it is the author’s opinion that they can be seen as fairly realistic. 40 Results 30 Applying the revised fishery mortalities led to the identification of the following year-classes as having a high number of immigrants from Greenland: 1936, 1937, 1938, 1942, 1945, 1950, 1953, 1956, 20 1961, 1962, 1963, 1973 and 1984 (Fig. 2). It is quite possible that some cod from other year-classes Catch in numbers (millions) 1945 might well have also migrated to Iceland, but if they 1944 existed those migrations would have been so small 10 that they could not be detected from the usual noise 1946 in the data. 0 Most abundant of all the Greenland year-classes 3456 789101112 Age (years) in the period in question was the outstanding 1945 year-class. According to landings from this year- Fig. 1. Cod at Iceland: catch-at-age for the 1944–46 class in the late-1940s and the beginning of the year-classes. 1950s, this year-class turned out to be of an aver- SCHOPKA: Greenland Cod at Iceland and Their Impact on Assessments 83 700 Icelandic cod Immigrants 600 500 400 300 Numbers (millions) 200 100 0 1938 40 45 50 55 60 65 70 75 80 85 Year-classes Fig. 2. Cod at Iceland: year-class size at age 3. age size at Iceland. However, suddenly in 1953 and years around 1950 when the actual stock biomass the following years, this year-class showed up in at Iceland was overestimated by 40-60%. When the large quantities on the spawning grounds off the 1973 and 1984 year–classes immigrated to Iceland, southwestern coast of Iceland (Fig.
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